RXJ1957.1-5738 - A NEW LOW-FIELD POLAR DISCOVERED WITH ROSAT

We report on observations of a new AM Herculis binary identified as th e optical counterpart of the X-ray source RX J1957.1-5738, detected du ring the ROSAT All Sky Survey. It has an orbital period of 99 min and is strongly modulated both in soft X-rays and at optical wavelengths. The source is bright for somewhat less than half the orbital period, s uggesting the disappearance of the accretion area behind the horizon o f the white dwarf for part of the orbit. During the bright phase the m ain contribution in the optical appears at red wavelengths which can b e explained as cyclotron emission in a relatively low magnetic field. The X-ray spectrum is dominated by a soft quasi-blackbody component, w ith a smaller contribution from thermal bremsstrahlung. The H alpha em ission line consists of a broad and a sharp component shifted in phase against each other. The broad component originates in the accretion s tream from the low mass companion star, the sharp component on the irr adiated surface of the low mass companion. The sigma(-), pi, and sigma (+) Zeeman absorption components of Her from a halo around the accreti on spot are identified and suggest a magnetic field strength of 16 MG. The distance to RX J1957.1-5738 is d similar to 350 pc.